In JP 2002-168947A, an FMCW radar device for detecting a physical quantity of a target object (e.g., a distance to the target object, or a relative velocity of the target object) also detects in two ways an occurrence of interference by another radar device.
In one way, the FMCW radar device compares, with a predetermined amplitude threshold, amplitude of a reception signal corresponding to a radar wave reflected by the target object or amplitude of a beat signal indicative of a frequency difference between a transmitted radar wave and the reflected radar wave. Then the FMCW radar device determines that the FMCW radar device is interfered with by another radar device, when the amplitude of the reception signal or the amplitude of the beat signal is larger than the predetermined amplitude threshold.
In another way, the FMCW radar device compares a peak frequency of the beat signal with a predetermined frequency threshold corresponding to the farthest end of a target object distance range within which the target object should be detected. The peak frequency of the beat signal is a frequency at which amplitude of the beat signal reaches a local maxim value. Correspondence between a frequency and a distance depends on how the frequency of the transmitted radar wave changes in time. Then the FMCW radar device determines that the FMCW radar device is interfered with by another radar device, when the frequency of the beat signal is higher than the predetermined frequency threshold.
The first way works by virtue of the fact that the interference increases amplitude of the reception signal and the beat signal increases because the FMCW radar device receives a radar wave transmitted from another radar device in addition to the reflected radar wave when the FMCW radar device is interfered with by another radar device.
However, in the case that a target object having a high reflectivity to the radar wave is located close to the FMCW radar device, the FMCW radar device possibly detects the reception signal with amplitude higher than the threshold amplitude. In addition, the amplitude of the reception signal is very high in an extremely low frequency range, because a fraction of the radar wave transmitted from the FMCW radar device always propagates, by diffraction, from an antenna for transmission to an antenna for reception.
It is thus difficult to make, simply based on whether the amplitude of the reception signal or the beat signal is larger than the predetermined amplitude threshold, a determination with high accuracy whether the FMCW radar device is interfered with by another radar device, because there are some causes, as well as the interference, which increase the amplitude of the reception signal.
The second way works by virtue of the fact that the peak frequency of the beat signal tends to be lower than the maximum frequency corresponding to the distance from the FMCW radar device to the farthest end of the target detection distance range, because the peak frequency of the beat signal is basically proportional to the distance to the target object. Thus, the FMCW radar device determines that the beat signal is generated in the existence of the interference by the other radar device, when the frequency of the beat signal is higher than the maximum distance frequency corresponding to the farthest end of the distance range in which the target object should be detected.
In the case that the FMCW radar device is installed in a vehicle, however, it often happens that a building, a large sized vehicle having a load on board or the like is at a location farther than the farthest end of the target detection distance range. If the building, the load, or the like has a wide area which is almost orthogonal to a propagation direction of the radar wave transmitted from the FMCW radar device and is capable of reflecting the radar wave, the FMCW radar device sometimes receives the reflected radio wave from the large obstacle such as the building or the load. The FMCW radar device therefore detects the beat signal having the peak frequency higher than the maximum frequency corresponding to the distance to the farthest end of the target detection distance range.
It is thus not only the interference by another radar device that causes the beat signal to have the peak frequency higher than the maximum frequency corresponding to the distance to the farthest end of the target detection distance range.
It is therefore difficult to make, simply based on whether the peak frequency of the beat signal is higher than the maximum frequency corresponding to the distance to the farthest end of the target detection distance range, a determination with high accuracy whether the FMCW radar device is interfered with by another radar device.